Pipe split, how does this happen?

I found what I thought was a pinhole leak in the 1 1/2" copper stub (attached to a pvc trap) under my daughters kitchen sink. When I cut just behind where I thought the leak was, I found the split you can see in the picture. I was wondering if this was a manufacturing defect or what can cause this sort of failure? This house was built in the 60s using 1 1/2" copper for DWV.